Theirs something I want to know, the name yellow card is it the name of the exchange?
Yes, it is a name of an exchange which we are discussing about on this thread.
Assuming I still use this exchange, this email would have been a warning sign for me and I would have stayed away from the exchange till further notice, at least when I know that everything has become functional again because this email seems like downgrading their services.
To maintain just the mobile apps will be easy than to mention them along with the website. I do not think what they did is wrong because everyone are now using mobile phone. Also, it is possible that they check the people means people's means of visiting their exchange and noticed mostly from mobile devices.